PBXs are increasingly being used in present day telephone systems. A PBX system ties together the telephones of an office, building or factory. Anyone within the PBX system can talk to someone else within the system without the cost and time of using outside lines and facilities.
Increasingly, PBX systems are becoming digital. The analog voice signals of a caller are converted into a digital representation. These digital signals are transmitted through the PBX system. Furthermore, PBX systems are increasingly used to transport computer data signals. This is due, in part, to the availability of personal computers in the home and office.
The heart of the system, the PBX switch as shown in FIG. 1, connects callers within the system, connects callers to outside lines if a call outside the PBX system is desired, and connects outside callers to lines within the system. A PBX switch generally has a number of modules or "line cards." Each line card is connected to a number of telephones or "terminals" and the line cards are connected to each other by a set of lines called a "bus", or sometimes, the "backplane bus." The bus, such as bus 10 in FIG. 1, has a timeslot bus. The timeslot bus carries the digital signals of a voice or the data of a computer, for example.
In a digital PBX, the voice signals are sampled at some rate, typically 8000 times per second (8 KHz), and the resulting voltage samples are converted into a digital representation, typically 8-bit ".mu.-law" or "A-law" encoding. The resulting sequence of bits (8000 times 8, or 64K bits/sec) is called the Pulse Code Modulation (PCM) representation of the original voice signal. The digital PBX transports and switches the PCM signals from place to place within the PBX system. Eventually, the PCM signals are converted back into an analog voice signal for a person to hear.
The PCM signals are carried on the bus 10 on which the signals are carried during particular time intervals, or timeslots. Each timeslot can carry the PCM 64K bit/second stream of data so that typically one timeslot is required for each incoming or outgoing voice path. Of course, a timeslot can also be used to carry computer data at rates up to 64K bits per second.
Besides a timeslot bus, the bus 10 has a signaling bus. Besides PCM-encoded voice signals and data signals, a digital PBX switch must also transport and switch "signaling" or control information associated with individual voice or data ports. For example, for a rotary-dial telephone it is important to know that the handset has been taken "off-hook," that a digit has been dialed, and so on. Thus, the PBX switch must have a way of gathering signaling information from individual voice ports, and transporting it to a control unit which acts upon this information by, for example, making voice connections.
All digital PBX switches must have a timeslot bus and a signaling bus of some kind. Associated with these buses are many conflicting goals and problems. Among these are:
(a) Universal bus and parallel bus wiring. A bus in a typical system has some number of "positions"; each position has a bus connector which mates with a module or line card connector to connect a module to the bus. A bus is "universal" if the same signals exist in the same positions in every bus connector in the bus. In a universal bus, any module may be connected at any position in the bus. The advantages of such a system are evident. PA1 (b) Maximum data transfer bandwidth for a given maximum signal speed. PA1 (c) Flexible timeslot allocation. PA1 (d) Individual module addressability. PA1 (e) Centralized or distributed timeslot switching.
A completely parallel bus topology satisfies the requirements of a universal bus. It is easily laid out in printed circuit board technology. Regardless of the number of positions in the bus, it can be easily connected at any point.
If just a few lines are not parallel, such as a star topology about the control unit, there are different lines at each potential breakpoint. Each position is different from the rest. The bus is no longer universal.
Several performance characteristics of a bus are limited by the maximum switching signal speed on the bus. For example, faster switching speeds limit the maximum bus length and also generate more radio frequency interference. On the other hand, faster speeds allow the bus to carry more information with a smaller number of wires. Therefore, with a given maximum switching signal speed, as many signals as possible should use this maximum speed to achieve maximum data transfer bandwidth.
Since different modules may service different numbers of voice paths, the goal of a universal bus implies that there should not be a fixed number of timeslots associated with any given bus position, even when centralized timeslot switching (FIG. 2a)) is used. Rather, timeslots should be allocated to individual modules as required by the particular system configuration.
Despite parallel bus wiring and universality, it is necessary to have some means of selecting individual line card modules for operations, such as sending and receiving signaling information, polling, and resetting. However, providing a separate "module select" line for each module violates the desired configuration of parallel bus wiring and resulting universality.
There are two different timeslot switching techniques that are used in existing PBXs. "Centralized" switching is shown in FIG. 2a). In this technique, there are logically two timeslot buses to carry voice and data signals. One bus carries outgoing timeslot signals from the central control unit to the line card modules containing the individual port circuits, and the other carries incoming timeslot signals, in the opposite direction. Each bus has a dedicated timeslot for every port in the system. For example, a voice port "X" always places its PCM signals on an incoming timeslot X, and receives PCM signals on an outgoing timeslot X.
Since the central control unit receives all incoming timeslot signals and transmits the voice or data signals on all outgoing timeslots to the line cards, timeslot-interchange circuits in the central control unit can make all connections. For example, to connect ports X and Y, the timeslot-interchange circuits in the central control unit are programmed to store the PCM samples that arrive on incoming timeslot X and transmit them on outgoing timeslot Y; and to simultaneously store the PCM signals that arrive on the incoming timeslot Y and transmit them on the outgoing timeslot X.
In "distributed" switching, shown in FIG. 2(b), there is logically just a single timeslot bus, and there are no centralized timeslot-interchange circuits. Instead, each line card module has a local timeslot-interchange circuit which can connect the incoming signals from any port to any timeslot on the timeslot bus, and which can also listen to the signals on any timeslot and send them to any outgoing port.
In this technique, to connect ports X and Y, the central control unit may allocate a pair of timeslots, say P and Q, which need not have any fixed relationship to X and Y. It then instructs the local timeslot-interchange circuit for port X to transmit on timeslot P and receive on Q, while it instructs the local timeslot-interchange circuit for port Y to transmit on Q and receive on P.
Typically, the choice of either centralized or distributed timeslot switching is based on the performance of the switching technique and the cost effectiveness of the technologies available at the time of the design. For example, the distributed technique utilizes timeslots more efficiently, since timeslots are not allocated for idle ports, while the centralized technique is typically less costly, since it requires just one timeslot-interchange circuit.
The present invention attains many of these goals and solves or substantially mitigates many of these problems above.